When an athlete first takes up a new sport she typically buys moderately priced equipment with decent performance characteristics. As her abilities improve she looks for ways to increase her performance and hence her competitiveness. Upgrading equipment is one method of increasing performance.
The vast majority of bicycles purchased are off-the-shelf bikes as opposed to custom bikes. A custom bike is created by buying individual components and assembling these components together into a complete bike. As a cyclist becomes more involved in the sport, it is typical for him to look to increase the performance of his bike. There are two ways to accomplish this goal. First, he can buy a whole new bike or maybe even create a new custom bike. Second, he can replace the “original equipment” components on his bike with higher performance custom or aftermarket components. For instance, a cyclist can replace his original equipment wheel with a lighter and higher strength model wheel such as the ZIPP 520 wheel.
Another bicycle component that impacts a bicycle's performance, and hence is often upgraded, is the bicycle's crank. The crank comprises the pair of arms which couple the bicycle's pedals to its chain ring.
There are four primary ways to increase the performance of a bike crank: (1) maximize power transfer from the rider to the wheel; (2) minimize weight; (3) reduce air resistance; and (4) increase reliability. Standard metal cranks are old in the art and have varying performance characteristics. Carbon fiber cranks are relatively new in the art and have far superior performance characteristics over metal cranks. Carbon fiber cranks are the state of the art in bicycle crank technology because they are stiff and light, and the fibers can be oriented in a direction to resist forces applied to the crank during cycling.
Most known “carbon fiber” cranks are not made entirely from carbon fibers. Rather, they often include one or more metal components along with carbon fiber components.
The material which makes up the carbon fiber component is usually comprised of a resin and carbon fiber composite material. The carbon fibers give the final crank added strength and stiffness that the resin by itself can not provide. Carbon fibers take on two distinct forms: chopped or continuous. In a chopped carbon fiber component, the fibers are relatively short in length and their orientation to each other is random. The presence of the chopped fibers increases the strength of the component. In a continuous carbon fiber component, the length of the fibers is relatively longer and their orientation to each other is structured. It is the structured orientation of the continuous carbon fibers that greatly increases the strength of the component. The continuous fibers can be oriented in a specified direction to better combat the effects of external loads imposed of a specific type, or from a specific direction.
Bicycle cranks are part of the transmission drive train of the bike. The cyclist exerts a load on the crank at the center of the pedal axis during a pedal stroke. This force varies between about 40 and 450 lbs depending on the riding conditions and strength of the rider. Initial pedaling, climbing or sprinting result in the greatest loads.
This force exerted by the cyclist when pedaling results in two types of deflections of the crank. The first type of deflection is a bending deflection that occurs about the bottom bracket in the connection point of the crank to the bike, in a plane parallel to the chain rings. This force is exerted generally in the direction of crank rotation. The second or torsional type of deflection is a twisting of the crank arm about an axis extending along the length of the arm from the pedal axis to the bottom bracket axis. The more the crank deflects torsionally, the less power is transferred from the rider to the wheel.
Compositech, Inc., the assignee of the instant invention, marketed the first continuous carbon fiber crank in the fall of 1997 under its ZIPP SPEED WEAPONRY trademark. The 1997 ZIPP crank is composed of three carbon fiber members. The first member is an elongated, generally ovaloid, hollow, continuous carbon fiber crank arm with a metal insert disposed at the distal (pedal) end and at the proximal (spindle) end. The distal end insert has a pedal receiving aperture and is over molded by the carbon fiber arm. The proximal end insert has a bottom bracket spindle receiving aperture and is secured to the carbon fiber arm with an adhesive material. The proximal insert is designed to couple the crank to a rectangular, 2° taper bottom bracket spindle. The proximal end of the carbon fiber arm receives a cap. The second carbon fiber member is a continuous carbon fiber spider that attaches the crank to the chain drive of the bike. The third carbon fiber member is a backing plate which secures the spider to the crank arm.
The 1997 ZIPP crank has several advantages over standard metal cranks. First, the hollow carbon fiber construction makes the crank significantly lighter than standard metal cranks. The 1997 ZIPP crank weighs only 350 grams. Second, the ovaloid shape of the arm is more aerodynamic than circular or generally rectangular cranks. Third, the 1997 ZIPP crank's ovaloid cross-section make the crank stronger than known metal cranks. Fourth, the continuous carbon fiber construction allows for the minimization of the rotational and torsional deflections the crank will experience during a pedal stroke. This optimization increases the life of the crank by reducing the amount of accumulative fatigue experienced. It also maximizes power transfer from the rider to the chain drive by reducing the bending and twisting deflections of the crank.
Although the 1997 ZIPP crank was the state of the art at the time of its creation, there was still room for improvement. One drawback with the 1997 ZIPP crank is its incompatibility with one of the more popular spline drive systems used on bicycles. Another drawback with the 1997 ZIPP crank is that it is difficult to manufacture, requiring the three separate components be molded out of carbon fiber, and then line welded to each other and bonded together. This process was labor-intensive and time consuming.
In order to overcome these difficulties, the assignee invented the 1999 ZIPP crank. The 1999 ZIPP crank includes a crank spider that is coupled to the second metal insert and bicycle chain drive, and a cap to cover the second end of the carbon fiber arm. The spider is replaced with a cover on the non-drive side crank. The drive side is defined as the side of the bicycle with the chain drive.
The 1999 ZIPP crank was manufactured by starting with a solid template whose shape generally matches the interior shape of the crank arm. The template serves as a base, around which uni-directional carbon fiber laminates are wound. Each laminate is wrapped around the template in succession culminating in the application of the outside or twill, carbon fiber laminate. At this point, the template is pulled out of the open second end of crank arm. The orientation of the carbon fibers is chosen to maximize the stiffness of the crank. The hollow interior significantly reduces the weight of the crank when compared to metal cranks.
A first insert is wrapped in a carbon fiber laminate and positioned in the interior of the crank arm at the first (distal) end. Upon curing the carbon laminates, the first insert is completely encapsulated by the crank arm at the first (distal) end.
After the resin has set, the crank arm is trimmed to its final length and a relief cut is made in the bottom bracket spindle facing surface of the crank arm at the second end. A second insert is then assembled to the crank arm by applying an adhesive to the interior surfaces of the crank arm at its second end, and inserting the second insert into the second end of the crank arm. A spider can then be attached to the second insert.
Notwithstanding the advantages obtained with the 1999 ZIPP crank (as compared with the 1997 ZIPP crank) room for improvement still exists. In particular, room for improvement exists in producing a crank design that is less time consuming to manufacture, and hence, is less expensive to produce, but which still retains the stiffness, aerodynamic properties, and light weight of the 1997 and 1999 ZIPP cranks.
One object of the present invention is to create a carbon fiber bicycle crank arm that maintains good weight, aerodynamic, and stiffness qualities, while being easier to manufacture than cranks heretofore known by the applicant.